In general terms, conventional oral preparations having high drug dose are hard to administer, because they tend to be bulky. A drug of low aqueous solubility has a difficulty in minimizing its size because certain design for the preparation is necessary for improving dissolution rate and the like to ensure sufficient efficacy. A drug having high water solubility is formed into tablets along with a small amount of excipients and the like, or the single crystal thereof is filled in a capsule to form a capsule preparation or in a dispense package to give a granule preparation. For production of tablets, some additives are necessary, but certain drugs do not permit addition of other substances in view of the stability. In this event, single crystal of the drug is directly filled in a capsule to form a capsule preparation or directly formed into granule preparation or powder preparation. The use of a drug in the form of single crystal leads to low flowability, which in turn produces variation in the amount to be contained in a capsule preparation or a folded piece preparation, thereby failing to ensure uniform content, or sometimes even filling itself. When packaged in a bottle and the like, an easy measure using a measure spoon and the like may become unattainable.
Generally, when a tablet preparation or capsule preparation containing a water soluble drug is orally administered; the blood concentration of the drug tends to increase in an initial stage and drastically falls thereafter. Such dissolution pattern of a drug does not maintain a desired blood concentration or efficacy of the drug. Thus, tablet preparation and capsule preparation of such water soluble drugs mostly fail to show sufficient potential of a water soluble drug, thereby limiting its clinical usefulness.
Starting from the 1970s, many attempts to improve drug release have been made by introducing various drug delivery systems. For example, it is now possible to prevent drastic increase in blood concentration and improve insufficient retention in the concentration range necessary for treatment, by controlling release of a drug from a preparation to a constant level. There are some preparations that have enabled a single administration per day of a drug having a short half-life time in the body, by controlling migration in the gastrointestinal tract. In addition, targeting of absorption site in the gastrointestinal tract and lesion has become available by a drug release at a desired time after intake and control of a drug release at a desired site in the digestive tract, thereby enabling administration in advance of a drug, avoiding the time period when drug intake is difficult. This in turn has afforded effective drug treatment while reducing side effects, improvement of QOL (Quality of Life) of patients as a result of reduced frequency of administration, and ensured drug treatment based on improved compliance.
On the contrary, the aforementioned techniques relating to preparations for oral administration defy easy application to water soluble drugs. In most cases, drug release of such drugs is controlled by the use of polymer matrix comprising a combination of various polymers and the like and film coating of various polymers and the like. In the case of the above-mentioned polymer matrix, however, the amount of a release control ingredient generally used has been insufficient to achieve suitable control of a drug release rate using a drug having extremely high aquenous solubility. To compensate this, the amount of a release control ingredient is increased, which problematically makes preparations bulky or large in size. In the case of the above-mentioned film coating, burst after administration has a risk of causing serious side effects because it is a single-unit type preparation.
Therefore, a multiple-unit type drug release control by coating various polymers and the like is considered to be more superior. However, the multiple-unit type release control requires increased amount of release control film coating agents to obtain a desired dissolution pattern of a water soluble drug. An attempt has been made to decrease the size of preparation by reducing the amount of additives other than the release control film coating agent. For example, direct coating of a single crystal of a water soluble drug has been reported. However, the use of a single crystal as it is is associated with difficulty in desirably controlling the release, because a single crystal of a water soluble drug has a large surface area due to its shape. To achieve the desired release control, the amount of a release control film coating agent needs to be increased, which prevents minimizing the size of preparations. In particular, the size of preparation and the like may pose a problem in the case of oral administration to elderly people. Thus, an oral pharmaceutical preparation free of such problems has been desired in clinical situations.